The present invention relates to a continuous rolling mill for producing seamless tubes from round intermediate forgings which are pierced axially and fitted onto respective mandrels, the rolling mill operating by the so-called restrained mandrel process.
In particular, this invention concerns a mandrel-carrier head used in a rolling mill of the aforesaid type for coupling a mandrel to an operating device which, if necessary, pre-inserts and then inserts the mandrel into an axially pierced intermediate forging to be rolled, pushes the mandrel between the rolls of the rolling mill along the rolling axis, retains the mandrel during the rolling of the intermediate forging, and finally recovers the mandrel when the rolling is finished. Such a device may be constituted, for example, by a racked rod which is engaged with one or more driven pinions and has an end portion formed in a suitable manner for firmly supporting a mandrel-carrier head.
In the structure of a mandrel-carrier head formed according to the prior art, there can be indentified basically a first portion for coupling to the rack and a second portion for coupling to an end part or tail of a mandrel. Usually, the second portion of the mandrel-carrier head has an essentially U-section widened in a direction parallel to the rolling axis, in which the limbs have different lengths and give this second portion and the entire mandrel-carrier head substantially a horizontal hook shape. The tail of the mandrel is also formed and shaped so as to engage or, better, hook onto the second portion of the mandrel-carrier head, and this hooking occurs automatically during positioning of the mandrel on the rolling axis upstream of the continuous rolling mill.
For a number of different reasons, such as, for example, thermal expansion of the mandrel, its deformation, unavoidable play and inaccuracies in the devices for transferring the mandrel, the inexact positioning of fixed abutments and/or stop devices, and other similar errors, this positioning is crude, that is to say, it may vary by several centimeters with respect to an ideal predetermined position. Consequently, in order to ensure that the automatic hooking always occurs, the distance between the limbs of the hook portion of the mandrel-carrier head is considerably greater than the length of the tail of the mandrel.
A mandrel-carrier head so constructed, however, has recognised technical disadvantages which have not been overcome until now. Indeed, when hooking has occurred, and the operations of inserting the mandrel into the intermediate forging, retaining the mandrel during the rolling, and recovering the mandrel are carried out, the tail of the mandrel knocks repeatedly and violently against the limbs of the hook portion of the mandrel-carrier head and may damage (for example, deform or even break) the mutually hooked parts.
A further disadvantage is constituted by the fact that the stresses and pressures acting on the mandrel during rolling may have vertical components, more or less accidentally, of such a degree as to cause the vertical movement of the tail of the mandrel with consequent unhooking thereof from the mandrel-carrier head. The serious consequences of such an eventuality are well known to those skilled in the art.
The problem behind this invention is that of devising and providing a mandrel-carrier head having structural and functional characteristics such as to overcome finally the disadvantages described above with reference to the prior art.